Approximately 50% of patients with autonomic failure (AF) suffer from supine hypertension, even those with very low plasma norepinephrine and renin. Because NO is arguably the most potent metabolic modulator of blood pressure, we hypothesized that impaired NO function contributes to supine hypertension in AF. However, we found that AF patients (n=14) were more sensitive to the pressor effects of the NO synthase inhibitor NG-monomethyl-l-arginine, suggesting increased NO function rather than deficiency; a lower dose of NG-monomethyl-l-arginine was needed to produce a similar increase in blood pressure in AF patients, as in healthy control subjects in whom AF was induced with the ganglionic blocker trimethaphan (171±37 mg versus 512±81 mg, respectively; P=0.001). Furthermore, potentiation of the actions of endogenous NO with the phosphodiesterase inhibitor sildenafil (25 mg PO) decreased nighttime supine systolic blood pressure from 182±11 to 138±4 mm Hg in 8 AF patients with supine hypertension (P=0.012 compared with placebo). Finally, AF patients tolerated a greater degree of upright tilt during infusion of NG-monomethyl-l-arginine (56±6° versus 41±4° with placebo; n=7; P=0.014), an improvement in orthostatic tolerance similar to that obtained with equipressor doses of phenylephrine. In conclusion, AF patients do not have NO deficiency contributing to supine hypertension. Instead, they have increased NO function contributing to their orthostatic hypotension. Potentiation of NO could be used in the treatment of supine hypertension, and its inhibition offers a novel approach to improve orthostatic hypotension.